1. Field of the Invention
The present invention relates to a current mirror circuit. In integrated circuits, current mirror circuits are used for example for realizing constant-current sources. In a basic circuit, a current mirror circuit has, in principle, an input path and an output path, which are coupled to one another. The input path generally contains a current source with which a transistor is connected in series by its main current path. The output path contains a further transistor, the control terminal of which is connected to the input path. Equally, the control terminal of the first transistor is connected to the input path.
A basic circuit of that type is described, for example, in Tietze, Schenk: Halbleiter-Schaltungstechnik [Semiconductor Circuitry], 10th Ed., Springer-Verlag, Berlin, 1993, pages 94-97. If both transistors are identical, in particular their width-length ratios, the same current flows through both transistors and thus through the input path and the output path. The current through the respective transistor is determined in particular by the gate-source voltage, and equally by the width-length ratio of the respective transistor. Given the same gate-source voltage, the magnitude of the current which flows through the transistors is generally proportional to its width-length ratio.
Current mirror circuits are used in particular as current sources for data receiver circuits, so-called data receivers. It is thereby generally desirable to operate the data receiver in a plurality of operating modes, for instance in a normal operating mode and a standby operating mode. The latter is characterized by a lower current requirement relative to the normal operating mode.
If the current of the current source, for instance, changes in a current mirror circuit, for example on account of a change in the operating mode, then, in particular, the gate-source voltage of the respective transistor in the input path and output path changes as a result. The temporal change is thereby dependent inter alia on line capacitances and so-called buffer capacitances. In the circuitry, comparatively large buffer capacitances are often used in order that the current of the current mirror circuit is kept constant in an operating mode and so-called noise is minimized. However, on account of long time constants that result, this leads to comparatively long changeover times for example when changing over from the standby operating mode to the normal operating mode.
It is accordingly an object of the invention to provide a current mirror circuit, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which, in the event of current changes in the current source, can be changed over for example from a normal operating mode to an operating mode with a reduced current requirement with comparatively short changeover times.
With the foregoing and other objects in view there is provided, in accordance with the invention, a current mirror circuit, comprising:
an input path with a current source and a first transistor circuit connected in series with the current source, the transistor circuit having a first transistor and a second transistor, and the second transistor can be connected in parallel with the first transistor;
an output path with a second transistor circuit having a first transistor and a second transistor, and the second transistor can be connected in parallel with the first transistor; and
the transistors of the first and second transistor circuits having control terminals that can be connected to the input path.
In other words, the objects of the invention are achieved by means of a current mirror circuit having an input path, which has a current source and, connected in series therewith, a first transistor circuit with at least two transistors, in which one of the transistors can be connected in parallel with the other of the transistors, having an output path, which has a second transistor circuit with at least two transistors, in which one of the transistors can be connected in parallel with the other of the transistors, and in which the control terminals of the transistors of the first and second transistor circuits can be connected to the input path.
The current mirror circuit according to the invention makes it possible to influence the gate-source voltage of the transistors in the input path and output path in the event of current changes in the current source by corresponding connection or disconnection of the connectable transistors in the input path and output path of the current mirror circuit. In particular, this can be controlled by corresponding connection or disconnection of the respective transistor in such a way that no fluctuations, or only comparatively slight fluctuations, of the gate-source voltage occur, even if the respective current changes in the input path and output path of the current mirror circuit. Consequently, there is no need to carry out charge reversal operations for example of line capacitances or buffer capacitances. This enables the current mirror circuit to be operated in two different operating modes, which differ in terms of the current requirement, with comparatively short changeover times.
For this reason, the current mirror circuit according to the invention can advantageously be used as a current source for a data receiver. These can be operated in a standby operating mode with a reduced current requirement, so that the power demand of, for example, an integrated circuit in the form of an integrated memory is reduced in this operating mode. With the current mirror circuit according to the invention as current source, the data receiver can be operated in the normal operating mode with a comparatively short changeover time.
In one embodiment of the current mirror circuit according to the invention, the connectable transistors of the respective transistor circuits are connected in the normal operating mode and disconnected in a standby operating mode. The disconnection of the corresponding transistor of the first transistor circuit makes it possible to ensure that even in the event of a reduced current in the input path, the gate-source voltage of the other transistor remains unchanged. In order to maintain the ratio of input current and output current, the corresponding transistor of the second transistor circuit is then likewise disconnected.
In accordance with an added feature of the invention, a ratio of the width-length ratios of the transistors of the first transistor circuit corresponds to a corresponding ratio of the width-length ratios of the transistors of the second transistor circuit. In one embodiment, the connectable transistors of the first and second transistor circuits have an identical width-length ratio.
In an advantageous embodiment of the invention, the current source is formed by a third transistor circuit, which has at least two transistors whose main current paths are connected to the input path, wherein one of the transistors can be connected in parallel with the other of the transistors. The current in the input path of the current mirror circuit can be changed through the connection or disconnection of the corresponding transistor of the third transistor circuit.
In accordance with a particularly advantageous feature, a ratio of the width-length ratios of the transistors of the first transistor circuit corresponds to a corresponding ratio of the width-length ratios of the transistors of the third transistor circuit. This enables the gate-source voltage of a transistor of the second transistor circuit to be influenced to the same extent as the change of the current in the input path through the third transistor circuit. In one embodiment, the connectable transistors of the first and third transistor circuits have an identical width-length ratio. In this case, through parallel connection and disconnection of identical transistors, the current in the input path and output path is changed without a change in the respective gate-source voltage of the transistors in the input path and output path.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in current mirror circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.